Recently, in the global automobile industry, the development of steels having excellent impact resistance and the application thereof have been in strong demand for weight reductions of a vehicle bodies in addition to passenger protection. In order to secure the above properties, research into ultra-strong steels having a tensile strength of 1300 MPa or greater has been actively undertaken, but it has been difficult to form automobile parts having complicated shapes therefrom, due to a lack of formability through having ultra-high strength and also to secure shape precision due to spring-back.
In order to solve the above problems, the Hot Press Forming (HPF) method has been suggested (for example, a technique suggested in U.S. Pat. No. 6,296,805 and the like). The technique presented in the above patent is a method for manufacturing an HPF part having ultra-high strength by heating a hot dip aluminum plated steel sheet, having thermal resistance at a high temperature, to a high temperature, forming a part by hot press forming, and then quenching the part to secure martensite throughout the entire part. However, the HPF part manufactured by the above technique is composed of martensite as a main phase in the entire thickness of the HPF part in a percentage of 100% to secure ultra-high strength. Sometimes, in the case that a cooling speed of a part in a mold is slow or the formed part is a thin plate (1 mm or less), ferrite and/or bainite may be formed at the martensite grain boundaries, but these microstructures cause a reduction of strength and bending properties of the HPF part. Accordingly, these microstructures are considered to be unintended microstructures.
Further, bending properties are considered as a representative index for evaluating impact resistance characteristics of the HPF part. For example, like a car B-pillar, fracture toughness, a measurement of steel properties determined by measuring a degree to which a car part can endure deformation without fracturing to a certain degree (angle) after the HPF part is bent by car side impact (bending properties), is required. However, it is known that the bending properties of HPF parts containing martensite and/or grain boundary ferrite throughout the entire thickness of the part may be degraded due to the presence of martensite, which degrades the ultra-high strength of the part itself and the bending properties structurally. When forming an HPF part to have a bainite microstructure in order to improve the bending properties, there is a problem that it is difficult to secure the ultra-high strength.
Thus, the present inventors have developed a method for securing impact resistance characteristic of the HPF part by improving bending properties thereof, as well as securing ultra-high strength therein.